Axial flow gas turbine engines employ longitudinally alternating arrays of rotating blades and nonrotating vanes in their compressor and fan sections. The blades and vanes cooperate to compress a stream of air flowing essentially longitudinally through the engine. It is not uncommon for the rotating blades, and particularly the fan blades, to each be formed with an integral shroud set. Each shroud set is a pair of circumferentially extending shrouds, one projecting from the suction surface and one projecting from the pressure surface of each blade. Because the shrouds are located intermediate the blade root and the blade tip, they are often referred to as midspan shrouds, although they can be located anywhere along the blade span, not just at the midpoint of the span. When all of the blades are assembled in an engine, the suction surface shroud of each blade abuts the pressure surface shroud of the neighboring blade so that the shrouds describe a ring. During engine operation, the shroud ring resists vibration and twisting of the blades.
During engine operation, boundary layers form on the engine surfaces, including the blades and shrouds. A boundary layer is a region or layer of impeded airflow adjacent the surface of any object over which the air moves. Air flowing in a boundary layer is dominated by the effects of fluid viscosity and flows with a lower velocity than the air outside the boundary layer, a region known as the free stream. Within the boundary layer, the air velocity is nonuniform, varying from zero at the surface to the free stream velocity at the outer edge of the boundary layer. The distance, perpendicular to the surface, over which the velocity varies from zero to the free stream velocity defines the boundary layer thickness.
At some operating conditions, the leading edge of each blade produces a planar shock wave that extends across the flow passages between the blades and crosses the suction surface shroud of the neighboring blade in the blade array. The properties of the engine air stream change abruptly as the air stream passes through the shock wave; for example, the free stream velocity decreases and the pressure increases. When the free stream airflow is decelerated by the shock wave, it loses momentum and a portion of the free stream air close to the slower moving boundary layer joins the boundary layer and becomes a part thereof. Consequently, while the blade and shroud boundary layers upstream of the shock wave are very thin, typically only a few thousandths of a centimeter, the boundary layers downstream of the shock wave can be quite thick. The thickening of the boundary layer is especially significant at the junctures between each blade and its suction surface shroud where the individual blade and shroud boundary layers combine into disproportionately thick, merged boundary layers. The merged boundary layer thickness, measured perpendicular to either a shroud surface or a blade surface, is as much as 15 % of the distance between neighboring blades, resulting in a considerable volume of impeded airflow.
The air in the blade and shroud boundary layers not only flows with impeded forward velocity, but is also centrifuged radially outward due to the rotation of the blades about the engine's longitudinal central axis. Consequently, when the air which flows in the boundary layer attached to the radially inner face of each shroud proceeds beyond the shroud trailing edge, it spills radially outward around the shroud trailing edge. The spilled air collides with and further impedes the slowly flowing, radially centrifuged air in the boundary layer attached to the shroud's radially outer face. The spillage is not usually harmful to the air flowing over the outer face of a pressure surface shroud. However the spillage encourages the air flowing in the merged boundary layer attached to the outer face of each suction surface shroud to separate from both the shroud outer face and from the adjoining blade surface. The airflow separation creates a region of high turbulence with attendant aerodynamic drag and degraded blade and shroud efficiency. A small region of such turbulence may be tolerable. However the disproportionate thickness of the merged boundary layer gives rise to a turbulent region of considerable volume and consequently, considerable drag.
The above described behavior might be mitigated by suction surface shrouds whose trailing edges project downstream of the blade trailing edges. In this case there is no juncture between the shroud and blade downstream of the blade trailing edge. The merged boundary layer, therefore, cannot extend to the trailing edge of the shroud, and hence cannot contribute to the volume of impeded, radially centrifuged flow near the shroud trailing edge. As a result, little or no separated, turbulent flow is created on the shroud outer face by the spillage from the inner face. However the extension of the shroud places unacceptable stresses on the thin trailing edge of the blade, and therefore is structurally untenable.
Therefore, it is seen that midspan shrouds of conventional construction, while beneficial for preventing blade vibration and twisting, diminish the operating efficiency of an engine. Accordingly, a shroud that provides its beneficial effects while minimizing aerodynamic drag and inefficiency is sought.